Micro-Electro-Mechanical Systems (MEMS) technology has been used increasingly in the development of many micro devices such as optical switches. MEMS technology utilizes lithographic mass fabrication processes used by the semiconductor industry in manufacturing integrated circuits (ICs). MEMS based switches consist of arrays of tiny mirrors and are found in two-dimensional (2D) and three-dimensional (3D) varieties. 2D MEMS based mirrors only can tilt in two positions, up or down, whereas 3D MEMS based mirrors can tilt in any direction.
Electrostatic actuators have been used to produce torsional motion of structures through the application of electrostatic force between stationary and mobile electrodes. The stationary electrode is usually attached to a substrate whereas the mobile electrode is attached to a torsional element or a torsional flexure, which is in turn attached to the torsional element. The flexure or mirror itself can serve as a mobile electrode if it is made from an electrically conducting material. The mobile electrode moves toward the stationary electrode once a bias voltage is applied between them. In known actuators, capacitive actuators that apply force directly on the torsional element itself provide a smaller rotational motion compared to the ones that apply force directly on the mirror's flexure. This is due to the larger gap size between both electrodes and the smaller angle of rotation that the mirror can have before it touches the bottom electrode in case of applying the force directly on the mirror itself.
FIGS. 1A-1C show perspective and cross sectional views of a prior art actuator 25 where the force is applied directly on the flexure itself. FIG. 1B shows a cross sectional view along line B of FIG. 1A, and FIG. 1C shows a cross sectional view along line C of FIG. 1A. The stationary electrodes 8 (FIG. 1C) apply the force directly on torsional flexures 4, which suspend the torsional element 2 and mirror 1 over a cavity 5. The mobile electrode 7 and torsional flexure 4 rotate together around rotation axis B and move toward the stationary electrode 8 upon the application of a bias voltage between both electrodes. The electrostatic force is inversely proportional to the square of the gap 9 between stationary 8 and mobile 7 electrodes while the maximum angle of rotation ∝ is directly proportional to the gap 9. An electrostatic actuator that applies force directly on the flexure itself is disclosed in U.S. Pat. No. 6,201,629B1 issued to R. W. McClelland et al.
In known electrostatic actuators, the voltage profile across the surface area of the stationary and mobile electrodes is uniform and its value is equal to the biasing voltage. Actuators that apply force directly on the mirror's flexure allow larger angles of rotation when compared to ones that apply force on the mirror itself. However, both types of actuators have non-linear actuation characteristics (i.e. non-linear variation of the angle of rotation with the applied voltage) and suffer from the conflicting demands of larger angle of rotation, lower actuation voltage, and higher switching speeds. In addition, both actuator types lacked the capability to create a non-uniform voltage profile across the electrode surface area and to dynamically change such voltage profile across the electrode surface area during operation. Providing such capability permits more versatile designs and/or larger rotational motion.
Therefore, there is a need for new types of rotating capacitive actuators and torsional micro-mirror systems to overcome the shortcomings of the prior art systems in terms of smaller size, higher resonant frequency, larger angle of rotation, lower actuation voltage, more precise position sensing, simpler fabrication methods, reducing the non-linearity of the actuation characteristics and providing a capability to dynamically change the voltage profile across the electrode surface area during operation.